Unleaded Gas

What is the Story on Lead Substitute Gas Additives?

by Paul Higley

I still add lead substitute to most of my older cars as I have not pulled the heads and replaced the valves and valve seats with those suitable for lead free gas. I always heard that the lead added to our gas left a thin layer of lead on the valves and seats. This improved the heat transfer and thus kept the valves from burning. Reading more on the subject, I find this is not the complete story.

Lead in gas had two effects: one was to slow the burn rate of the gas and thus raise the octane rating. This prevented pre-ignition known as knocking or pinging and permitted the use of higher compression ratios. The second was to protect against very rapid wear, or “recession” of the exhaust valve seats.

The lead additive of choice was tetra-ethyl lead (TEL). TEL was first added to gas during the 1920’s as an anti-knock additive. It was not well understood until later that it formed deposits which acted as a solid film lubricant on exhaust valves and seats. These deposits were useful when the engine was run hard and the valve temperature rose. Apparently at very high temperatures the valves micro-weld themselves to the seats. When they open a fraction of a second later, they pull iron particles from the valve seat. These particles are exposed to hot exhaust gas and are not well coupled to bleed off the heat. The exposed iron particles heat to extreme temperatures and form iron oxide particles which become imbedded in the valve. Iron oxide is harder than the metal in older valves or valve seats. This hard coarse surface causes the valve to start wearing into the valve seat. As the valve rotates the imbedded iron oxide causes increased wear in the valve seat. This is called valve seat recession. The TEL additive formed a lead compound that coated the valves and seats. This coating both conducted the heat well and prevented the micro-welding and thus the oxides from forming. Without the TEL and when the engine is run hard, the valve temperature rises. The longer and harder the engine is run the more rapidly we get rapid recession of the valve seat. Worse yet, as the valve recesses, and assuming you do not reset the tappet clearance, the valve spends less time seated and heats even more. This causes more oxide to form and the wear rate to increase. The seat continues to wear and eventually the valve becomes so hot it burns.

So how do we prevent our older cars from damage? We have several choices.

1. Do not drive on extended runs or drive the car hard. Well that leaves out touring in Texas and participating in autocross so that is not really an option at all. Ray would have to park his Spitfire and not do any 100 mph testing on his way to Houston.

2. Rebuild the head and install new valves and valve seats. This is a good option if you are in need of a head rebuilding anyway and it is completely effective. It is, however, an expensive option for a car otherwise running well. It is not an option for some of the older cars which will not accept replacement valve seats or require custom produced valves.

3. Buy lead substitutes and add some to each tank of gas. These lead replacement additives usually contain some combination of phosphorus, sodium or potassium compounds. This is a low cost option and lead substitutes are readily available at local auto stores. Hopefully this will be the case for years to come. However there is one warning that should be added. According to some testing done in Australia and in the UK, the effect of lead substitute additives is very temporary. While the effect of TEL in protecting your valves lasts several tanks of gas past the last use, the effect of phosphorus, sodium or potassium additives is gone almost immediately when gas without the additive is burned. So if you use any of these additives, be sure to be regular in its use.

As for the original reason for adding lead based TEL to our gas, we need to worry about replacing the anti-knocking qualities of leaded gas. The octane rating of our gas now is boosted by the addition of ethanol. I just need to replace fuel pump diaphragms, rubber fuel lines, all the rubber gaskets in my 32 and 49 and any paint on which I spill this new ethanol laden gas.